1. Field of the Invention
The present invention relates to a sheet feeding apparatus and an image forming apparatus, and more particularly, to a sheet feeding apparatus having a sheet feeding portion that separates and feeds sheets one by one and an image forming apparatus having the same.
2. Description of the Related Art
In the related art, there is known an image forming apparatus such as a printer having a sheet feeding apparatus configured to store recoding sheets in a sheet cassette and separates and feeds the stored sheets one by one. This sheet feeding apparatus has a nip guide member that guides a leading end of the sheet to a separator for separating and feeding sheets one by one, and the nip guide member can move as the sheet bundle bumps (refer to Japanese Patent Laid-Open No. 2003-118865).
Hereinafter, a configuration of the sheet feeding apparatus of the related art will be described with reference to FIGS. 18, 19A and 19B. FIGS. 18, 19A and 19B are cross-sectional explanatory diagrams illustrating the sheet feeding apparatus of the related art. FIG. 18 illustrates a state before a feed operation is performed. FIG. 19A illustrates a state when a single sheet S is fed to the separator. FIG. 19B illustrates a state when sheets S are fed to the separator as a bundle.
Referring to FIGS. 18 and 19, the sheet feeding portion has a pickup roller 130 and a pair of separation rollers 134. The pair of separation rollers 134 includes a feed roller 131 and a retard roller 132 arranged to face the feed roller 131. The retard roller 132 is pressurized by a spring (not illustrated) toward the feed roller 131 with a predetermined contact force.
The feed roller 131 is configured to control rotation and stop using a feed clutch (not illustrated). As the feed clutch is turned on, a rotational driving force in a sheet feeding direction (arrow direction “a” in FIG. 18) is transmitted the feed roller 131 via a feed roller shaft 131a. A rotational driving force opposite to the sheet feeding direction is transmitted to the retard roller 132 via a torque limiter (not illustrated) supported by the retard roller shaft.
A nip guide member 62 is provided in order to prevent a sheet S from being trapped between a sheet cassette 133 and a pair of separation rollers 134 to generate a jam during a feeding operation. The nip guide member 62 receives a force to approach the feed roller 131 (in the arrow direction “b” in FIG. 18) by a tension spring 64 by setting the rotation shaft 62b as a rotation center.
As illustrated in FIG. 18, the nip guide member 62 is positioned by a stopper 63 with a predetermined angle θ (where 0<θ<90°) with respect to a feeding direction (the arrow direction “P”) of sheets S loaded on a sheet supporting plate 141 which is pushed up by a rotational arm 143. As power is transmitted to the pickup roller 130 and the pair of separation rollers 134 so that the pickup roller 130 abuts on a sheet top surface, and a single sheet S is fed, the sheet S is guided to the separation nip portion N by the nip guide member 62 as illustrated in FIG. 19A.
When a single sheet S is fed, a load is not nearly applied from the sheet S to the guide surface of the nip guide member 62. Therefore, the nip guide member 62 abuts on the stopper 63 without rotation as illustrated in FIG. 19A.
However, the number of sheets S fed by the pickup roller 130 is not limited to one. A plurality of sheets S may be fed from the sheet cassette 133 as a bundle. In this case, a significant load is applied to the nip guide member 62 from a bundle of sheets S. As a result, as illustrated in FIG. 19B, the nip guide member 62 rotates with the rotation shaft 62b as a supporting point to recede from the feed roller 131 (the arrow direction “d”). It is noted that a significant load is also applied to the nip guide member 62 if a sheet such as a thick sheet having high rigidity is fed even when a single sheet S is fed from the pickup roller 130.
As the nip guide member 62 rotates in the arrow direction d, an angle θ between the nip guide member 62 and the sheet bundle increases. Therefore, a bundle of sheets S is loosened and guided to the separation nip portion N. It is noted that the sheet separated in the separation nip portion N is fed to a pair of conveying rollers 136 and is conveyed to an image forming portion.
As described above, in the related art, an angle θ between the nip guide member 62 and the sheet S changes depending on whether a single sheet is fed or a bundle of sheets is fed. In addition, if a sheet S having high rigidity such as a thick sheet is fed even when a single sheet S is fed, the angle θ between the nip guide member 62 and the sheet bundle changes. In this manner, as the angle θ changes depending on the number of sheets or stiffness of the sheet, conveyance resistance of the nip guide member 62 for a sheet S also changes.
For example, in the case of a thick sheet having high rigidity or a sheet S having a rough cutting surface, as the conveyance resistance of the nip guide member 62 increases, a sheet leading-end may be trapped in the nip guide member 62 and fail to advance to the downstream side to generate a delay. Recently, there is a high speed tendency in printers. Therefore, if there is a delay in conveyance of a sheet S in a sheet feeding apparatus of a high-speed printer, a jam may occur due to a conveyance delay (a sheet may fail to reach a predetermined location within a predetermined time).
In the technique of the related art, when a bundle of sheets S or a sheet having high rigidity is fed, the nip guide member 62 is retracted, and an abutment point of the sheet leading-end on the circumferential surface of the retard roller 132 changes.
In particular, responding to a miniaturization tendency in recent printers, a diameter of the retard roller tends to decreases. In this case, when the abutment point of a sheet leading-end on the retard roller changes, an abutment angle between the sheet leading-end and the circumferential surface of the retard roller (an acute angle between a tangential line at the abutment point of the sheet leading-end on the retard roller and the sheet) abruptly increases. A circumferential surface of the retard roller has a high frictional coefficient. Therefore, if a sheet leading-end bumps into the retard roller with a large abutment angle, the sheet leading-end may be significantly damaged, or a jam may occur as the sheet leading-end fails to enter the separation nip portion N.
The present invention provides a sheet feeding apparatus and an image forming apparatus capable of reliably separating and feeding sheets one by one to a downstream side without a delay and damage to the sheet in order to respond to a high speed, miniaturization, and applicability to various media of a printer.